In recent years, a strong demand has been created for increased density of magnetic recording. A perpendicular magnetic recording method has been used as a technique for realizing high-density magnetic recording. A perpendicular magnetic recording medium based on such a method includes at least a nonmagnetic substrate and a magnetic recording layer including a hard magnetic material. In addition to the abovementioned components, a perpendicular magnetic recording medium may further optionally include a soft magnetic underlayer that is formed from a soft magnetic material and serves to concentrate the magnetic flux generated by a magnetic head in the magnetic recording layer, an underlayer for orienting the hard magnetic material of the magnetic recording layer in the target direction, and a protective layer that protects the surface of the magnetic recording layer.
A granular magnetic material can be used for forming the magnetic recording layer of the perpendicular magnetic recording medium. The granular magnetic material is constituted by a magnetic material and a nonmagnetic material and has a granular structure constituted by magnetic crystal grains and a nonmagnetic material that has precipitated such as to surround the periphery of the magnetic crystal grains. In recent years, for example, ordered alloys have attracted attention as the magnetic materials, and carbon based, oxide base, and nitride based materials are known as the nonmagnetic material (see, for example, Patent Literature 1).
Various materials have been suggested as granular magnetic materials for perpendicular magnetic recording media, but where the thickness of the magnetic recording layer increases, the phenomenon of the nonmagnetic material precipitating not only on the grain boundaries of magnetic crystal grains, but also on the surface of magnetic crystal grains, and inhibiting the growth of magnetic crystal grains can occur, depending on the combination of an ordered alloy as magnetic crystal grains and a nonmagnetic material (see Non-Patent Literature 1). Where the thickness of the magnetic recording layer is further increased after such precipitation on the surface has occurred, the so-called secondary growth occurs in which the magnetic crystal grains grow on the nonmagnetic material that has precipitated on the surface of magnetic crystal grains. Such secondary growth results in the decreased magnetic anisotropy of the perpendicular magnetic recording medium.
Accordingly, various nonmagnetic materials have been studied for inclusion in granular magnetic materials. For example, Patent Literature 2 discloses a perpendicular magnetic recording medium including a magnetic recording layer using B4C as a nonmagnetic material. Patent Literature 2 reports that high thermal stability and a high magnetic anisotropy constant (Ku) can be realized and that the film is grown by a DC sputtering method. However, Patent Literature 2 does not mention using B4C in combination with an ordered alloy.
Patent Literature 3 discloses a heat-assisted magnetic recording medium having a magnetic layer of a two-layer structure in which a granular magnetic material, in which an oxide such as SiO2 is added as a nonmagnetic material to an ordered alloy having a L10-type crystal structure, is used for the first magnetic layer, and a continuous layer (CAP layer) including no such oxide is used as the second magnetic layer. The specific feature of the invention disclosed in Patent Literature 3 is that the content ratio of the nonmagnetic material in the first magnetic layer decreases from the substrate side toward the second magnetic layer, and because of such a configuration, the excess nonmagnetic material is prevented from precipitating on top of crystal grains of the ordered alloy and the growth of crystal grains is prevented from splitting in the perpendicular direction. As a result, fine crystal grains of the ordered alloy that has grown continuously in the direction perpendicular to the substrate surface are realized.
A magnetic recording medium constituted by a plurality of magnetic layers including a granular magnetic material, in which the concentration of a nonmagnetic material differs between the magnetic layers, is also known.
For example, Patent Literature 4 discloses a method for manufacturing a perpendicular magnetic recording medium including a step for forming a substrate temperature controlling layer on a substrate, a step for forming an underlayer on the substrate temperature controlling layer, and a step for forming a magnetic recording layer on the underlayer, wherein in the step for forming a magnetic recording layer, a magnetic layer stacking step, which includes a first step for heating the substrate in a heating chamber and a second step for producing, in a film production chamber, a magnetic recording layer constituted by an alloy based on FePt additionally containing at least one nonmagnetic material selected from the group including C and Si oxide, is repeated N times (N≧2 times). With such manufacturing method, variations in the substrate temperature during the production of the magnetic recording layer are reduced and a crystal grain size is decreased. Further, in Patent Literature 4, the amount of the nonmagnetic material added to the magnetic recording layer is larger on the substrate side.
Patent Literature 5 discloses a magnetic recording medium having a magnetic recording layer constituted by a plurality of magnetic layers including a magnetic alloy containing Fe and Pt as the main starting materials and at least one nonmagnetic material selected from carbon, oxides, and nitrides. In the magnetic recording layer disclosed in Patent Literature 5, the second magnetic layer positioned of the surface side with respect to the first magnetic layer has a homogeneous structure constituted by a mixture of crystal grains that are finer than the FePt magnetic alloy grains of the first magnetic layer (the layer constituted by the FePt alloy and the nonmagnetic material). Further, in Patent Literature 5, the first magnetic layer is characterized in that the content of carbon, which is a nonmagnetic material, is greater and the content of oxides and nitrides, which constitute other components, is less than those in the second magnetic layer.    Patent Literature 1: Japanese Patent Application Publication No. H8-083418    Patent Literature 2: Japanese Patent Application Publication No. 2008-097824    Patent Literature 3: Japanese Patent Application Publication No. 2011-154746    Patent Literature 4: Japanese Patent Application Publication No. 2012-048784    Patent Literature 5: Japanese Patent Application Publication No. 2012-181902    Non-Patent Literature 1: Appl. Phys. Express, 101301, 2008    Non-Patent Literature 2: Fuji Jiho, vol. 83, No. 4, 2010, p. 257-260    Non-Patent Literature 3: R. F. Penoyer, Rev. Sci. Instr. 30 (1959), p. 711    Non-Patent Literature 4: Kyojiseitai-no Butsuri (part 2), Chikazumi Soushin, Shokabo, p. 10-21
In a perpendicular magnetic recording medium, a high magnetic anisotropy should be maintained. Further, in a perpendicular magnetic recording medium, it is desirable, that the grains of the ordered alloy in the granular structure be refined (see Patent Literature 3). Furthermore, magnetic recording media for energy-assisted recording, such as heat-assisted and microwave-assisted recording, has recently attracted attention as the magnetic recording media with increased recording density (see Non-Patent Literature 2). According to the research conducted by the inventors, it is preferred that a magnetic recording layer in a magnetic recording medium for energy-assisted recording have a certain thickness. Thus, a magnetic recording layer for a perpendicular magnetic recording medium is required to include a fine-grain ordered alloy and have a predetermined thickness while maintaining a high magnetic anisotropy.
However, with the ordered alloy-nonmagnetic material combinations that have been conventionally used, a sufficient thickness cannot be realized with a monolayer configuration. Further, even among the magnetic recording media including a plurality of magnetic recording layers constituted by a granular magnetic material, a magnetic recording medium in which a predetermined film thickness is realized while maintaining a sufficiently high magnetic anisotropy could not be heretofore obtained.